MetService keep close eye on tropical cyclone

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Social media is lighting up with early comments from Nelson surfers excited about the possible effects of a cyclone that could approach from the north Tasman Sea.

The MetService is taking a more conservative approach, but said it was "keeping its eye on it".

"An area of potential development over the Coral Sea has been highlighted by meteorologists as an area to watch over the coming days," meteorologist John Law said.

He said conditions over the Coral Sea looked favourable for the formation of the next tropical system.

"It is still too early to say how much of an impact it will have on us in New Zealand, but the forecast team are keeping a very close eye on it," Mr Law said.

Nelson surfer Mike Baker said it initially looked like Nelson might get the kind of "epic surf" surfers hung out for from one January-February to the next, but now, it was looking like the storm's swell-generating easterly edge would fall over the northeast quadrant of New Zealand.

MetService media and communications meteorologist Daniel Corbett said a swell could potentially develop from the brewing tropical low, but it would become clearer what might happen over the next 24 hours.

Tropical cyclones forming in the Coral Sea fall under the responsibility of the Australian Bureau of Meteorology (bom.gov.au) which gave forecasts and warnings as required.

Mr Corbett said so far the bureau was saying that there was a moderate risk of the system developing into a "full-blown tropical cyclone", but it was too many days away to really know and whether it might have any impact on New Zealand.

Mr Corbett said the ingredients needed for a cyclone to form were water temperatures of 26.5 degrees Celsius and warmer, which occurred in the tropics, a "low sheer environment" and outflow.

"As it draws warm moisture from the water, it needs outflow, much like a chimney.

"As these systems come closer to New Zealand water temperatures get cooler so they weaken, but they can still cause heavy rain and flooding.

"We will certainly be watching by Monday, Tuesday and Wednesday next week," Mr Corbett said.

Mr Baker said his reading of the upcoming weather "could be a pipedream".

"Right now, the system is not even on our map, and allowing for the vagaries of weather in the Roaring Forties, the whole thing may just miss New Zealand altogether. This is what happens when, as a surfer, you think out loud on Facebook," Mr Baker said.